docs: keep only bilingual (English + Simplified Chinese) README and CONTRIBUTING

Remove ja/ko/ru/zh-TW translations of README.md and CONTRIBUTING.md.
The tracker workflow now only syncs docs/README.zh-CN.md; language links
in README.md, CONTRIBUTING.md, and the AGENTS.md layout are updated to
the two maintained languages.

Co-Authored-By: Claude <noreply@anthropic.com>

CONTRIBUTING.md

@@ -90,12 +90,10 @@ Any readable commit message in semantic format is welcome. Such as:
 
 ## Translations
 
+Only English and Simplified Chinese are maintained:
+
 - English
-- [Simplified Chinese](docs/CONTRIBUTING.zh-CN.md)
-- [Traditional Chinese](docs/CONTRIBUTING.zh-TW.md)
-- [Russian](docs/CONTRIBUTING.ru.md)
-- [Japanese](docs/CONTRIBUTING.ja.md)
-- [Korean](docs/CONTRIBUTING.ko.md)
+- [简体中文](docs/CONTRIBUTING.zh-CN.md)
 
 ---